Useful articles from various thermoplastic resins have been prepared from molds for many years. Injection molding provides a convenient way for preparing various articles from thermoplastic resins, particularly objects of a relatively intricate nature. In order to injection mold articles in an economic manner, the mold resident cycle time should be kept to a minimum. This shorter cycle time provides a shorter resin heat period with consequently less thermal damage to the resin itself and/or less thermally promoted interaction between the resin and various additives present in the resin. In order to accomplish a release of the resin from the mold, various mold release agents have been found which provide for a release of the resin with lower ejection pressure. Such an agent should be chemically compatible with the resin as measured by the usual characteristics of the resin under normal conditions and heat treatments.
Additionally, it is also helpful to provide plasticization if at all possible. By plasticization is meant better melt flow of the thermoplastic resin. This plasticization should be preferably achieved without any significant lowering of any of the physical properties of the resin. With respect to polycarbonates, examples of the physical properties which should be substantially unchanged are light transmission, impact resistance, delamination and toughness of the material.
Esters prepared from monocarboxylic acids and multihydric alcohols have long been known as mold release agents for polycarbonate. For example U.S. Pat. No. 3,784,595 discloses the use of esters of trihydric alcohols and aliphatic carboxylic acids with 10 carbon atoms or more at levels of 0.1 to 2 wt. % of polycarbonate. DE2729485 discloses the use of esters of tetrahydric alcohols and carboxylic acids with 6 carbon atoms or more for mold release of polycarbonate at levels of 0.1 to 3 wt. %.
Aliphatic acid esters as plasticisers for polycarbonate have been disclosed in U.S. Pat. No. 3,186,961. Only esters having up to 22 carbon atoms and derived from C.sub.2 to C.sub.18 aliphatic carboxylic acids and C.sub.1 to C.sub.6 monohydric alcohols are disclosed. The only working examples of the esters disclosed in the patent are directed to aromatic esters. Furthermore a very special process for compounding these materials in sufficiently high concentrations is suggested. In the DE2729485, and U.S. Pat. No. 3,784,595, the major examples utilizing the multihydric alcohols were with the ester prepared from stearic acid. In the DE patent, the highest weight percentage of ester actually employed in the examples was 0.3 wt.%.
It has now been discovered that the usage of lower carbon atom aliphatic acid esters than employed previously brings about substantially better mold release for polycarbonate than the stearic acid esters of tri and tetrahydric alcohols, particularly pentaerythrityltetrastearate. This particular tetrastearate has been used commercially as a mold release agent for polycarbonte since 1977. Furthermore, at higher levels of the invention ester, very good plasticization of the polycarbonate occurs without significant deterioration of physical properties.